It would depend if that £15k took you into a higher tax bracket though. If you get into 40% tax then with NI contributions as well it's going to leave you with just over half the money. As you say £500 a month id assume you've gone slightly over the threshold, so part of the £15k is taxed at 40% and part at the lower rate.

Query with your manager / HR if it seems incorrect. It might have to do with the effective date of the salary increase (i.e your salary could have increased part way through the pay period). If this is the case, the next pay packet should be as you expected.

Depends on what tax bracket you will fall in. Could be tax (40%), national ins (9%) and pension (?) so you will be looking at over 50% being taken out. If you go with 55% then you will be left with 6750 which is about £553 per month extra take home.

40% tax does not start at £32k, it starts at around £42 as you have around £10k of tax free income, so if you were earning say £40 and went up to £55 the most of the increase would be taxed at higher rate.The salary calculator mentioned above has been invaluable to me when considering which job to take.

Student loan contributions will also increase (if you are paying one) so you have to factor in increase in tax, ni, pension and student loans. Enter the figures into www.listentotaxman.co.uk and that should tell you what your take home pay should be.